Antarctic Peninsula warming: natural variability or “global warming”?

Most people know that the Antarctic Peninsula is one of the most rapidly warming places on earth. But like everywhere else in Antarctica, the length of available temperature data is short — most records begin in 1957 (when stations were put in place during the International Geophysical Year); a few start in the late 1940s. This makes the recent rapid warming difficult to evaluate; in general, what’s interesting is how the trend compares with the underlying variability. As anyone who’s been there can tell you, the weather on the Antarctic Peninsula is pretty wild, and this applies to the climate as well: year to year variability is very large. Put another way, the noise level is high, and discerning the signal requires more data than is available from the instrumental temperature record. This is where ice cores come in handy — they provide a much longer record, and allow us to evaluate the recent changes in a more complete context.

A new paper in Nature this week presents results from an ice core drilled by the British Antarctic Survey (BAS) at James Ross Island on the Antarctic Peninsula. Before discussing the results, congratulations are due to Rob Mulvaney and his team for obtaining the ice core in the first place, quite apart from the analyses. James Ross Island is a gorgeous place to work on a sunny summer day, but it can be brutal on a bad day, and the BAS team spent many months in the field. The record they obtained is some 50,000 years long — by far the longest record ever obtained on the Antarctic Peninsula. (James Ross Island’s ice cap was originally cored by a Argentine/French team back in the 1980s, but they obtained just a 400 year record).

Figure 1.The glaciated summit of Mt. Haddington, site of a new ice core record reported by Mulvaney et al., as viewed from Santa Marta Cove, James Ross Island, on a nice summer day. (photo: E. Steig).

So do Mulvaney et al.’s results allow us to discern signal from noise? The short answer is yes, but it’s not that simple. This is quite apparent from the reporting on the paper, most of which has been pretty accurate, yet with headlines running the gamut from saying that the cause of recent warning is “unclear” (NPR) or “part of longer trend” (Australian Broadcasting Corp.), to “most warming in Antarctic is human caused” (Climate Central) and a more subdued “ususual but not unique” from the BBC.

Figure 2. Temperature anomalies averaged over the Southern Hemisphere (top), temperatures from the sub-Antarctic station Orcadas (middle), and hydrogen isotopes (a proxy for temperature) from James Ross Island on the Antarctic Peninsula (bottom). All the data are publicly available: GISS, SCAR, Nature

Why the ambiguity? After all, the results show that it’s now warmer on James Ross Island now than it has been at any time during at least the last millennium (see Figure 2), and its unequivocal that this recent warmth led to the demise of ice shelves in the area over the last few decades. Moreover, the rate of recent century-scale warming is at the upper limit of rates in the pre-anthropogenic era: Mulvaney et al. find that the most recent warming is faster than 99.7% of any other given 100-year period in the last 2000 years. Why then, doesn’t this lead simply to the conclusion that this that recent warming and associated ice shelf collapse and glacier acceleration on the Antartic Pensinula is the result of human activities?

Well, as we’ve noted on manyprevious occasions, attribution of climate change to specific causes is not simply a matter of looking at whether a particular year or decade is “exceptional” or not. Mulvaney and coauthors have been careful, both in their paper and in interviews about it, to avoid the suggestion that their results are any sort of “smoking gun” pointing towards an anthropogenic cause of the recent warming trend on the Antarctica Peninsula. As I tried to make clear in my News&Views commentary that accompanies their paper, I largely agree. Century scale warming trends comparable to that of the last century have happened before, without any influence from humans, and if we didn’t have any data other than this one ice core, we really couldn’t say much more than that.

What’s missing from this argument, of course, is that we do have other data. For one thing, recent rapid warming in Antarctica isn’t limited to the Peninsula. Whatever you may have read in some quarters, the borehole temperature data from WAIS Divide show definitively that West Antarctica is warming too — and in recent decades at least, the rate has been comparable to that on the Peninsula. More importantly, warming is occurring on the Peninsula at the same time that most of rest of the planet is warming. As the figure illustrates, in the Southern Hemisphere that warming has been nearly monotonic since about 1920, whether one is talking about the Southern Hemisphere as a whole (top panel of the figure), at James Ross Island (bottom panel), or at Orcadas in the South Orkney Islands, the one and only location south of 60°S that has a century long instrumental temperature record. This makes it much harder to argue that it is merely “local variability” that explains the recent warming trends. The James Ross Island results thus add yet one more bit of evidence to what we already knew: global warming is.. well, global.

Certainly, this is not the last word. A fly in the ointment here is that at least part of the most recent warming trend may be anthropogenic, yet not due to greenhouse gases in the troposphere. As many studies have argued (see e.g. the review by Thompson et al.), the stratospheric ozone hole has caused changes in the winds around Antarctica, and one of the consequences appears to be advection of warm air from the north onto the Antarctic Peninsula, especially on the east side (where James Ross Island is) during summer. Of course, the ozone hole didn’t exist before the 1970s, so it clearly can’t be invoked for the warming since the 1920s, but any formal attribution study will need to take this, as well as “global warming”, into account.

Mulvaney et al.’s paper doesn’t claim to be an attribution study; it’s largely just reporting the data, which is entirely appropriate. And nor am I suggesting that the simple comparison I’ve made above is any sort of formal attribution study either. Only one formal attribution study has been done for Antarctica that I’m aware of — that of Gillett et al., 2008 — and that was before any of the recent results showing warming in West Antarctica, and doesn’t including any of the longer term data from ice cores. What’s needed now is an updated study, taking into account all the available data, including that from ice cores. I would very much like to see someone take this on. Including the new results from James Ross Island will be an important part of such work.

Great stuff. I would love to see this in compared to a paleo record of Antarctic frontal positions or ice discharge. I would expect some kind of dynamical response to the reported 20th century warming, even if the mechanism is not straightforward. Such paleo evidence could probably help constrain Antarctic mass loss projections.

[Response:One aspect of the paper that I didn’t mention — though it’s in my N&V — is that the last time James Ross was probably as warm as today is about 1900 years ago. That’s the same time that Prince Gustav Channel (which became glacier-ice-free in 1995) was open. Pretty good evidence of the temperature/ice sheet dynamics linkage we see today applies on longer timescales. Which is no surprise of course. –eric.]

But please note that the second graph tends to be blocked by Adblock Plus. Also the page (and entire site) looks very narrow on Firefox (with large font for a High-Dpi screen). It looks somehow better in Chrome and IE.

Why have you only looked at the last 1500 years? Is it possible that the MSM outlets have come to their conclusions by looking at the whole record?

[Response:The “MSM outlets” haven’t come to any conclusions; they’ve just reported what we and others have said, but put slightly different “takes” on it. As I noted above, the recent warming is similar to the warmth of about 2000 years ago. I cut off the graph at 500 A.D. so that the instrumental data wouldn’t be too squished to the right on the graphs. You can look at the figures in the paper yourself if you like — previews are available even without a subscription to Nature. Keep in mind that on long timescales, Milankovitch forcing comes into play, so it’s really not very relevant to the global warming question whether it was warming or cooling on thousands of year timescales. –eric]

Your final observation is interesting – that (anthropogenic) global warming is having an effect on the WAIS despite a cooling trend which would have been expected as a result of stratospheric ozone depletion. Penetration of the Southern Ocean by warmer Pacific and Atlantic currents are of course not affected by the ozone hole but atmospheric warming should be – shouldn’t it?

[Response:I agree with your general point about competing trends, but overall, our work has shown that the ozone-forced trends are totally overwhelmed by other forcings, at least in West Antarctica. Note that in any case, there’s little evidence that the ozone hole should have led to any strong temperature change on the West Antarctic ice sheet. Those arguments and evidence apply most to the East Antarctic and only in summer, but the biggest trends over WAIS (and the Ant Pen too) are in winter, spring, and to some extent fall. Even in East Antarctica, my view is that the ozone-related forcing has been greatly overstated. For example, the ‘type locality’ for cooling trends — the South Pole — is actually not cooling. The 60-year trend is flat.–eric]

Eric, thanks for the good article (and a great addition to the proxy archive by the authors):

When I get the time I will track back to the relevant references, but for now I will be lazy and just ask:

How well accepted is it that (in this region) the spatial slope of the dD-T relationship is representative of the temporal slope (and constant over the Holocene)? This is pretty critical for this sort of study, and I see that the authors justify their assumption somewhat (particularly with the deuterium excess relation) but all my graduate work in the tropics has sort of embedded the philosophy into my head that we don’t understand the controls on isotope variability very well.

To be sure, the controls on tropical variability seem to be tied a lot closer to deep convection, rainout upstream, seasonal source changes, etc. I’ not well read in this region of Antarctica though. It could be nice to introduce an isotopic-enabled modeling component into this type of stuff.

[Response: Hey Chris, good to hear from you as always. My brief response is twofold. 1) I think people make too much about the oxygen isotopes as a temperature proxy, for which they are imperfect for sure. But oxygen (or hydrogen) isotopes are a perfect proxy for… oxygen isotopes! And those isotope ratios are still exceptionally high in the last few decades, relative to the last millennium. That’s a very robust result that does not depend at all on whether the isotope ratios “really” reflect temperature change. And to change the isotopes, you have to change climate — e.g. circulation sea ice, etc. So it is still a significant climate change, corresponding with the global warming trend. Indeed, it may well be more meaningful than temperature change, because temperature can change locally due to rather small changes in radiative balance e.g. albedo; whereas isotopes in precipitation are a good integrator over larger spatial and temporal scales. The problem is, no one but a few specialists think of isotope ratios as any sort of fundamental “climate state variable”, so we translate into temperature mostly because we all have a more experiential understanding of it. 2) In any case, the difficulty of relating isotope changes to temperature in the tropical regions really doesn’t apply in the polar regions, where the relationship is both much simpler and much better understood. For example, the borehole temperature reconstructions from WAIS Divide are in just as good — if not better — agreement with the isotope ratios as with the estimated surface temperature histories. Louise Sime, a coauthor on the Mulvaney et al. paper, has done a very careful job of evaluating this for the Peninsula, and it really is very robust there. — eric]

Regarding Milankovitch forcing, I’m puzzled. There was an overall cooling trend since the early Holocene until the recent reversal. Yet, the precession cycle has been giving the region increasing summer insolation during the period, hasn’t it? What accounts for the observed cooling?

The take by the denial crowd centers on the statement that temperatures were 1.3˚C warmer 11,000 years ago. The implication, I believe, is that the ice didn’t melt then, so it won’t now, and today’s temps are no big deal.

My presumption is that just because temperatures on the peninsula were warmer, that doesn’t mean they were uniformly warmer everywhere, in a way that would melt the ice as we’re already seeing today.

Another take would be that they are right (to a degree), that a warming of another 1˚C will not melt much of Antarctic ice (although shooting past a total of 2˚C warming would obviously still not be a good idea).

Can you elaborate on this? Does the paper explain the differences between then and now?

[Response: The first question I’d ask is where is the evidence the ice didn’t melt then?–eric]

Yes, I guess to some extent it’s a question not of if but how much. I had to revisit the ages of the ice shelves, and certainly it seems that Larsen A and maybe B did melt in that time frame, so I guess the answer is pretty non-controversial, as much as some people seem to think it’s somehow ground-breaking.

I can’t find ages for Larsen C, or the larger shelves, like the Ross or Filchner-Ronne.

The denial tack then becomes, I suspect, that not that much ice melted before (from a sea level rise POV), so no worries now. Again, “it’s happened before, so what’s the big deal?”

I can’t find the paper to see what this is about. Does anyone have a link?
11kya – think about the aftermath of the Younger Dryas and changes in the Laurentide ice sheet. What was the global temperature?

Sphaerica @ 13 & 14 – around 11,000 years ago global sea levels were rapidly rising, but by 8000 years ago had begun to slowly grind to a halt. This work by Mulvaney and c o. is broadly consistent the global sea level trend as indicated by the deglaciation models.

“But the new record may not convince some global warming skeptics, who continue to question the general scientific consensus that human-generated greenhouse gas emissions are helping drive climate change. Just last week, a team of British Antarctic Survey researchers published findings in the journal Nature that recent warming in the Antarctic was “unusual” but not unprecedented, since ice core samples showed the continent experienced a warm period several thousand ago, and temperatures had begun to rise again 600 years ago after a relatively cool period.”

[Response: The ‘temperatures began to rise 600 years ago’ is a bit unfortunate, and really not meaningful, as is apparent from the graph (e.g. Figure 2 above). As I said in my various interviews about this, the question is what is the spatial fingerprint like? If there were evidence it has been warming globally for 600 years, that would be something. It’s also a bit strange that less emphasis was put in Mulvaney paper that the recent decades are warming than anything in the last ~2000. I didn’t realize that myself until after the paper was published and I had access to the data.–eric]

[blank lines added for online readability]
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Abstract: The equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115.000 years ago. The difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones infered from reconstructions for the last glacial inception.

This is a major improvement over previous similar studies, and the increased realism is attributed to the higher spatial resolution in the atmospheric model, which allows for a more accurate representation of the orography of northern Canada and Siberia.

The analysis of the atmospheric heat budget reveals that, as postulated by Milankovitch’s hypothesis, the only necessary positive feedback is the snow albedo feedback, which is initiated by reduced melting of snow and sea ice in the summer.

However, this positive feedback is almost fully compensated by an increased meridional heat transport in the atmosphere and a reduced concentration of low Arctic clouds. In contrast to similar previous studies the ocean heat transport remains largely unchanged. This stability of the northern North Atlantic circulation is explained by the regulating effect of the freshwater import through the Nares Strait and Northwest Passage, and the spiciness import by the North Atlantic Current.

The fact that the realistic difference in snow accumulation is achieved with the same model that is used for the fifth assessment report builds trust in the ability of climate models to anticipate the evolution of climate in the future.
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“Spiciness” is “… a variable that describes how hot and salty (‘spicy’) water of a given density is.” (Scholar turned that up too)

DP @22 — Actually not. Consider the two ‘points of accumulation’ of the Laurentide ice sheet to observe how far from the ocean those were. Siberia lacked an ice sheet simply becuase the winds didn’t blow that way.